The invention relates to a method of manufacturing a body of cerium-doped quartz glass by melting together silicon dioxide and a cerium compound in a reducing atmosphere.
The invention further relates to cerium silicate having the composition Ce.sub.2 Si.sub.2 O.sub.7.
The invention also relates to a lamp envelope of cerium-doped quartz glass and a lamp having an envelope of UV-absorbing quartz glass.
Lamp envelopes of quartz glass are used for various types of lamps in which high temperatures occur during operation of the lamp, for example for mercury-vapor lamps and halogen incandescent lamps. In various applications it is desirable to absorb light having a short wavelength, for example UV light. This can be achieved in a simple manner by doping quartz glass with, for example, an oxide of a rare earth metal, such as cerium.
Cerium-doped quartz glass and the lamp and lamp envelope to be manufactured therefrom are described in U.S. Pat. No. 4,361,779. For doping quartz glass with cerium, in general, cerium (IV) oxide CeO.sub.2 is used, see for example U.S. Pat. No. 4,798,681 in which the manufacture of luminescent quartz glass is described, this quartz glass comprising, inter alia, cerium in the form of trivalent ions. The preparation of quartz glass often takes place in a reducing (mostly hydrogen-containing) atmosphere to counteract the formation of bubbles in the finished product, see for example the above-mentioned U.S. Pat. No. 4,361,779. The use of the known method to manufacture cerium-doped quartz glass, however, leads to the formation of water as a result of the reaction: EQU 2CeO.sub.2 +H.sub.2 .fwdarw.Ce.sub.2 O.sub.3 +H.sub.2 O
which causes the finished product to exhibit too high a water content. The presence of water leads to a high IR absorption and, hence, to a high temperature of the quartz glass during operation of the lamp. The water present also adversely affects the gas content of the lamp and a reaction of the water with the electrode or the filament of the lamp may lead to blackening of said lamp. These effects reduce the useful life of the lamp. Cerium (III) oxide Ce.sub.2 O.sub.3 cannot be used as an alternative to the doping of quartz glass because this oxide is not stable in air, thus causing problems during doping (quantity) and mixing. Other cerium compounds, such as cerium aluminate, are not suitable when the presence of foreign metal ions in the quartz glass is not desirable.